Lithium ion batteries are secondary batteries, which are characterized by high energy density and possibility to obtain relatively high voltage, and multiply used for compact electronic devices such as a notebook computer, a video camera, a digital camera and a mobile phone. Also, it becomes promising as power sources of large equipments such as electric automobiles and distributed-type power sources of families in the future.
As anode active materials used for the lithium ion batteries, the conventional lithium composite oxides such as LiCoO2, LiNiO2 and LiMn2O4 are representative, and among them, the LiMn2O4 having a spinel structure is excellent in safety and advantageous in cost because it uses manganese, which is rich resource. Accordingly, it is receiving the attention as an anode material for a lithium ion battery.
However, the LiMn2O4 caused structural distortion called Jahn-Teller distortion resulted from Mn3+, and when using a manganese oxide containing Mn3+ was used as an anode active material for a lithium secondary battery, its life characteristic was bad at high temperature of 55° C. or more due to manganese elution.
Many technical improvements have been studied to overcome these problems. For example, in order to improve cycle characteristic of LiMn2O4, a method for substituting a part of Mn in the LiMn2O4 with hetero atoms was known. Japanese Patent Laid-Open Publication No. H11-189419 (Patent Reference 1) disclosed that, in a lithium manganese composite oxide having a spinel structure and expressed by composition formula Li1+xMn2−yMyO4+z, 16 d site is doped with a trivalent metal such as Co, Cr and Al, and the doping with the trivalent metal is very useful because it controls capacity reduction at a minimum. However, this substitution method had a limit because it could not reduce Mn3+ ions abundance.